A description of the seasonal variations of water exchange between the Baltic Proper and the Gulf of Bothnia

MERENTUTKIMUSLAITOKSEN JULKAISU

HAVSFORSKNINGSINSTITUTETS SKRIFT N:o 215

A DESCRIPTION OF THE SEASONAL VARIATIONS OF WATER EXCHANGE

BETWEEN THE BALTIC PROPER AND THE GULF OF BOTHNIA

ERKKI PALOSUO

HELSINKI 1964

Helsinki 1964. Val tioneuvos toil kirjapaino

Introduction

Numerous studies have been published on the transport and exchange of water in the seas around Finland. The first extensive study was made by Witting (1908). His data, collected at fixed stations, were mainly measurements of currents taken at an- chored lightships and tide-gauge readings.

The temperature and salinity values of the open sea were obtained during four cruises made in 1904 and 1905—the spring cruises in May and the fall cruises in August — September.

Notable among the later studies were the joint Swedish-Finnish cruises to the Åland Sea and the Archipelago Sea in the summers of 1922 and 1923. One of the main points of the program was the measurement of cur- rent systems. Palm i (1930) computed the mean current vectors for the surface water and published these as a current chart. The observational data were mainly current speeds measured on lightships. The Åland Sea was again studied by Hela (1958), who made a computation of the distribution of currents on the basis of temperatures and salinities measured during a special cruise in 1956. The Quark was studied by Li-sitzia (1946), who compared the currents in the Quark with variations in water level and other factors.

As mentioned above, Witting was able to perform two cruises in each of the years 1904 and 1905, a spring cruise and a fall cruise.

During the following years no vessel was available for marine research, except for too short a period in summer. This meant that the oceanographic sections could only be traversed once a year as a rule, and al- most always in summer. Thus it was im-

possible to study the seasonal variations in the deeper parts of the sea, especially as winter observations were almost entirely lacking.

As the advective transport of heat proved to be of great importance in the calculations of the cooling and freezing of the sea areas, the freezing calculations being of the utmost importance to winter navigation, arrange- inents were made in 1959 to use Finnish icebreakers for oceanographic studies in connection with their regular work. In 1961, coast-guard cutters also made observations at oceanographic stations. The latter opera- ted mainly in the Åland Sea and in the southern part of the Bothnian Sea, but also in the Bothnian Bay. In 1962, it was possible to use the research vessel Aranda for one early and one late cruise in the Gulf of Bothnia. This was repeated in 1963.

Since the data collected are by no means simultaneous, only a quantitative survey is attempted.

In this connection the author wishes to thank the masters and crews of the ice- breakers and coast-guard cutters who have assisted in the sampling program for their cooperation. The samples were analyzed at the laboratory of the Institute, under the direction of Dr. Folke Iforolejf. Therefore my thanks are directed to him and other colleagues. I wish to thank especially the Director of the Institute, Prof. Ilmo Hela, for his invaluable advice on the treatment of the data. Mr. Ilklea Noponen and Osnio Ranta-alto have helped with the drawing of the figures and Mr. Svante Nordström and Mrs. J. ll1. Perttunen helped with the trans- lation and checking of the manuscript.

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Contents

Page

Introduction... 3

I. The flow of water over the southern sills of the Åland Sea ... 7

II. The deep current from the Åland Sea to the Bothnian Sea ... 11

III. The flow of water through the Archipelago Sea ... 15

IV. The

water flow in the upper layers of the Bothnian Sea ... 17

V. The flow over the Quark to the Bothnian Bay ... 24

19° 20°

60° 6Ö

19° 20°

Vi qC

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knan„; • • _alskär

~ Yxsjkärs

•

• • Sommard • .

• ~

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Pi•i.I ~

_ y • r Isn;; i..~~Ö

Susoskn Hiigllrnnl

s e. ian i

Fig. 2. The depth contour-s o/ the Ålland ,Sea.

Td'hite: 0— 20 to

Light grayish: 20— 60 ett

Gray: 60-100 Sta

Dark gray. 100 -200 ni

Vety dark gray: more titan 200 ni

Black: land

I. The flow of water over the southern sills of the Åland Sea

The exchange of water between the Bal- tic proper and the Gulf of Bothnia proceeds on the one hand through the rather large and deep Åland Sea, and on the other hand through the numerous but shallow sounds and channels of the Archipelago Sea.

From the geological point of view, the Åland Sea is a tectonic depression of appre- ciable depth (Fig. 2). It consists of two basins more than 200 in deep, both with shallower margins and separated by a sill.

The southern basin, called Lagskär deep, is separated from the Northern Baltic by a ridge, over which the water depth is 40 m.

This ridge is partly cut by a deep but narrow canyon, the depth of which ranges from 150 to 225 m; however, at its southern entrance the depth of this canyon is only 70 m. The only attempt known to the author at measuring currents in the canyon indicated a rather strong northward current close to the bottom.

The sill between the two basins of the Åland Sea consists of the island of Lågskär and the shallow water to the southwest and east of this island. The deepest place on the sill, to the southwest of Flötjan, is about 80 m.. East of Lågskär the sill is cut by a very narrow canyon, but most of the water exchange must occur west of Lågskär be- cause of the wider opening.

The northern basin of the Åland Sea, the Aland Sea proper, is over 200 m deep. At Understen the trench grows narrower and continues into the Bothnian Sea as a chan- nel with a sill depth of some 80 m.

At the southern boundary of the area stu- died lies a lone rock with a lighthouse, Bog- skär. Just off this rock is a deep depression,

the Bogskär Deep. It has been impossible to make frequent visits to this out-of-the-way place for oceanographic measurements. Fur- thermore, old serial observations covering the whole year exist for the Bogskär Deep.

The Bogskär lighthouse was manned from 1898 until the destruction of the tower in 1914, and the lighthouse crew made depth observations every ten days (Witting, 1908).

The year 1907-08, shown here as an exam- ple (Figs. 3-4), indicates that the winter turnover reaches a depth of 80 m, this col- umn of cold water being of rather low sal- inity, 6.35-6.50 0/00. In summer the sal- inity increases at greater depths, being 7.00-7.25 °/oo at the end of July even at 50 in and higher than 8.00 °/oo below 70 m, as was also shown by Simojoki (1946) in his study of Bogskär data.

The saline water mass close to the bottom at Bogskär has been sampled during the recent cruises, and the oxygen concentra- tion of this water has always proved to be low. Thus in 1961 the oxygen concentration at 85 in was 1.73 ml/1, corresponding to 21 per cent of saturation.

The oceanographic station F 69 of the Lågskär Deep has been visited occasionally not only by R/VT Aranda, but also by the coast-guard cutters (Fig. 34). The varia- tions are similar to those at Bogskär. The largest fluctuations of salinity take place in the layer between the surface and the depth of 80-90 m. Below 100 m the salinity in- creases abruptly and reaches values higher than 8 0/oo. This water seems to be more or less stagnant.

In the Åland Sea the coast-guard cutters carried out numerous observations at dif-

491

150

III

F ^____

ui

! , Ii'ili! _WI! ^t , , '

^vI

_iIIiipIi

Fig. 3-4. Temperature and salinity at Bogskcir, in the Northern Baltic in 1906 to 1908.

8

ferent stations in 1962 and 1963 (11Ierenttutk.

J2tlk. No. 208). Since all stations show sim- ilar features, the annual cycle of the selected station, FÅ 4, will be analyzed (Fig. 5-6).

At this station, too, the annual cycle is remi- niscent of that at Bogskär. In summer the surface layer of warm water of low salinity, extending to a depth of about 20 m, appar- ently flows southward most of the time (shitting and Pettersson 1925, Hela 1958).

The water close to the bottom, presumably flowing more or less steadily northwards, is of rather high salinity. The maxinnan sal- inity is attained at this station in early Sep- tember, i.e., some weeks later than at Bog- skär.

In the fall, when the surface water has become cold enough, vertical convection be- gins to penetrate deeper and deeper. In De- cember 1961 the 6-degree isotherm sank as low as 125 in and the 5-degree isotherm to 195 m, almost to the bottom. At the time when the sea ices over, in late January or early February, water of 0°C reaches to a depth of 70 in and that of 2°C to 120 in.

In the spring, water colder than 2°C is found at a greater depth than 120 ni. This water, with a salinity of about 7.00 probably comes from the Northern Baltic, where it is found as a nearly homogeneous layer from the surface to a depth of 50 in.

It is interesting to make a comparison be- tween winters of different types. During the extremely mild winter of 1960/61 (Meren- teLtic. Julle. No. 200) hardly any cold water

appeared in the Aland Sea. The winter of 1961/62 was colder, and the Åland Sea and the Bothnian Sea had an ice cover for a short period (11Ie•entattk. Julle. No. 206.);

cold deep water was observed for rather a short time only. The winter of 1962/63 (1lierentutk. Julle. No. 213) was very cold;

even the Northern Baltic had an extensive ice cover for a long period. That winter the cold deep water of the Åland Sea did not appear until the spring, when the ice cover broke up in connection with the spring storms.

The amount of dissolved oxygen (Fig. 35) supports the above view. In the winter at station

rÅ

4 the concentration approaches saturation. This moans that even the deep water is derived from well mixed surface layers not too far away. In the summer the amount of dissolved oxygen decreases in the deeper layers, the saturation percentage dropping to 80 or 70. It is not impossible that part of this depletion of dissolved oxy- gen is caused by advection of deep, less oxygenated water from the northernmost part of the Baltic proper.

2 6464-64

200

Fig. 5-6. Temperature and salinity fluctuations at Station Pil 4, in the Aland Sea, isa 1961 to 1963.

150 50

1961 1962 1963

0 vm Ix x xl x11 I 11 III Iv v vl vii VIII Ix x xl x11 I II 111 IV v VI vII. vill / 50

55d I ₁ 600 _{ 5251 I i `°ill .,—

650 '

700 , l —65

, 1

4

'/// %

.

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/// /

k

l%

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%

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AIAND SEA

I / ; 700 '

/

7.25 -7.50 1 j / 7.50 //// FÅ4

// , // // I Y/ ”" S 0

f9725

i / 700// /////// /ir//

/

150

200

1963

I III N \/ vi ,/1I VIII

•110

>/2a +f2

/ 1 1

; 0~t~~',li„6

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.I

_`—,, ^{I \ / I}

ÅLAND SEA

II

I ///.

I,I 'I 1{

1

>.3 FA 4

I i // / 1 1 ^I I I I / ^i/ '/ /,I _>e2

L

^{1, 1/ / ' I > p I , /~ ////1 1 <2,,}

- +~i I I ;4 ~4 a1 ///2

10 I

II. The deep current from the Åland Sea to the Bothnian Sea

As mentioned above, close to the bottom conditions of the Åland Sea (Witting and of the Åland Sea there must exist a north- Pettersson 1925, Lisitzin 1951) the velocity ward current. In a study of the summer of this current was found to be 10 cm sec–'.

1962

Il Ili . Iv . v . vI . VII . VIII . Ix . x . XI . XII

>+ 7 ^{+10 +i +2 ,/ i / b} '.~+2111:6 1~;;``os\ ^{13 1U+ 6 2} f ,1 _i 111 _{1 1} 1 11 11 - _k y~ I11, _II

t ^{,1' I} 11 1111

/,♦~

^{nl 1} ₁ ¹

// _1~

///, t 1 (// '

11 II /~

~~' Y~~~

I I GRUNDKALLEN

>+ I ~i ^{' F33}

/ ' i f® C I I II I i I

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< 25 •

\

J

⁵ _{1 \} ^6.00

1k% 650

1 w6.00, I 1

I ~

/ I I ~ 1 ' // GRUNDKALLEN

›zoo 1.6.50'

; F 33

Z 6.75 6.75 I 700 7. / 7.

Fig. 7-8. Ten I peratlere (1 5d salinity Ili?etUatio?is at Station F 33, at

»Gncnzdkallell1> in 1961 to 1962.

50

1961 1962

0 vm ix x XI XII. I Il Ili IV v vl VII VIII Ix x XI xl1

50

j11

50

1961 1962 1963

0 VIII IX X XI XII I II III IV V VI VII VIII IX X XI XII I , II III IV V VI VII VIII

j 600

i

5

__

`~ SEA OF BOTHNIA~

6. ` _.6.50

Sn^eo

// `

,>,700 / 6.75-7.00 ~,

'700 6.75 f I 6.75 6.75

12

The observations in the canyon at Grund- kallen at station F 33 (Fig. 7-8) show an annual cycle similar to that at station FÅ 4 in the Åland Sea (Fig. 5-6). The northward current seems to continue through the whole canyon. It is interesting to observe that in 1962 the maximum salinity was at- tained at F 33 before the middle of Septern- ber, which is — very roughly — some ten days later than in the middle of the Åland Sea, FÅ 4. This compares well with a cur- rent velocity of 10 cin sec-1.

In the winter of 1961, the turnover of water at Grundkallen seems to have ex- tended to a depth of 60 m. At greater depths low temperatures were observed, but the salinity increased with depth. Obvious- ly, this higher salinity is connected with a northward advection from the Åland Sea.

On its arrival at the Bothnian Sea the northbound deep water will follow the deepest trench of the sea as a result of its relatively high density. This means a flow towards the northeast and then, starting at

50

We

>+11 1. 111, `` +1~i i~ 1,1 11111 I1111111~15

.1 ' `/ ~fj j~l II III ~ ~~`i~ \~~` ~

\S////// `\ SEA Ör BOTNN/A " 11 ///////Z// ~ 'ii/' i;` 7 o F30 ~1

E%»jIl4 4 < •p/Y / moi'

Fig. 9-10. Temperature and salinity /litctuationls at Station F 30,

in

the Bothnian Sea., in 1961 to 1963.

13

the latitude of Rauma, almost due north.

This is seen from the observations at F 30, west of Rauma (Figs. 9-10). Here an in- crease of the salinity is observed in the deepest parts of the water column in the fall, maximum salinity appearing in Octo- ber. Advection over the distance from F 33 to F 30 in one month would correspond to a speed of some 4 cm sec -'. This corre- sponds quite well with other computations (Voipio, personal communication).

The cold water of the Northern Baltic, being always connected with the winter con- vection, penetrates to the greater depths with some time lag, for obvious reasons.

During the short winter of 1962 (111erentuti . Julle. No. 206), only a limited amount of cold deep water was formed. This water was found at the bottom of F 30 as deep water from May to July. In the severe winter of 1963, the whole Åland Sea and the Baltic Sea as well were iced over as early as Feb- ruary and the ice cover did not melt away until the end of April (112erentutic. Julle.

No. 213). That year the cold deep water did not appear at F 30 until later, at the end of May or early June, and was still there in September, indicating either large amounts of cold water, or slow advection, or both.

Iyo I ^{IJO~ qwo}

VIII IX , X XI XII ^{1 II III IV} V VI VII VIII IX X XI XII I II

1 1 ' ffi✓ ~I ~ ~ ,~~ I ~,~

i

^{' ' ~} . ^{/ '} NORTHERN QUARK '1 I / /'/ ' / I/ / / i IIII

4 -3 il i/ z' / / .2l I ,5 q 4 1/1/

1961 1962 1963

0 Vm Ix x XI xn I u III IV v VI VII Vm ^{Ix x XI} XII I II

50

100

Fig. 11-12. Te» iperature and salinity /luetuatio>zs at Station F 18, at

»Sydostbrottcn> in the Quark, in 1961 to 1963.

0

50

100

<5. 5 575 5.75'%•'y' 4.00

'\

__i_ "

^X5.25

X550

å 5.75

•' ^6.00 NORTHERN QUARK

_\^_.

~ — _I F 18

- S %()

6.50 d25

14

In the northern part of the Bothnian Sea, at the latitude of Ulvö and Storkallegrund, only a few winter observations have been made at F 24 for practical reasons.

Just south of the Quark observations have been carried out at F18 (Figs. 11-12), close to the position of the Swedish light- ship Sydostbrotten. The opportunities for actual winter observations were limited.

However, taking into account that the whole water column is homogeneous and rather cold throughout the winter and that most of it stays rather cold all summer, possibly indicating a cessation of the north- bound current. This deep current, charac- terized by the cold, relatively saline water, must have taken more south another course, presumably towards the west and even south. Simultaneously, only small changes are observed in the salinity and oxygen concentration (Fig. 37).

In connection with the autumn turnover, in October through December, the coldest water seems disappear from the deeper part of the water columns.

Summarizing, it may stated that the ob- servations thus show two different kinds of water flow from the Åland Sea to the Bothnian Sea: the saline summer water and the cold winter water, both along the bot- torn. The former is derived from a depth of some 50 to 70 m at the latitude of Bogskär and the latter from the surface, perhaps in the same region. The flow follows the trench of the Bothnian Sea from Grundkallen to the latitude of Storkallegrund, where the saline summer water is thought to flow westwards towards the area more than 200 m deep off Ulvö. This water rarely.

if ever, seems to penetrate further north- ward.

III. The flow of water through the Archipelago Sea

The whole Archipelago Sea is rather shal- what deeper (30 to 35 m) channels, Norra low (Fig. 13), there being only two some- Skiftet and Delet.

28 ') 21

-- --- — — -- -- -- ISOKARI

H4 60.

e<~t r 1 4 4~ ~, TURKU

ENKLINGE` . '` k: •

' ~ dry ~:O` rwi (lig 1. * • fil

;

r,s ~.,• 7i '~• LON ~~~Y~y 1 S~~J x

~ lom•'

I1 ai

` 8EA'GISKAR.;

Jn •i 71 2)3Q 22 2210'

Pig. 13. The depth contours o/ the Archipelago Sea.

White: 0— 20 in Light grayish: 20— 60 ni

Gray: 60-100 In

Bloc/c: land

M

4.00

5.00

6.00

16

Fig. 14. The ckaii yes in Ilie dle>isily o/ sXi:jace water at Jungjrntskäv in the A>:chippelago ,Sen, a>id oj deep water oj F 30 in the Bothnian Sea.

Current measurements were made by the Swedish-Finnish joint expedition in the summers of 1922 and 1923 (1Pittin.g and Pettersson 1925). According to the measure- ments analyzed by Paln?fn (1930), the re- sultant current of nearly 10 cm see' in the whole Archipelago runs clue north both at the surface and at the bottom. Sjöblom (1961) also made some measurements in winter 1953 and the results confirm Pal- men's conclusions.

Oceanographic observations have been made at the island stations of Ruotsalainen, Skiftet, Eniklinge and Jungfruskär (Aferen•- tntll. Julin. No. 201). Since these stations all give a consistent picture, Jungfruskär, as the site of the most extensive data, has been selected for the purpose (Fig. 14). These comprise a set of temperature and salinity

observations for a number of depths every tenth day. Furthermore, the observations at station P ^30, in the south of the Bothnian Sea, have been used, although they com- prise only scattered observations anade on R/V Aranda, on the icebreakers, and on the coast-guard cutters in 1962.

The density values computed from tem- perature and salinity observations show that in winter the Jungfruskär water is heavy enough to sink to depths greater than 40 m or even 80 in in the Bothnian Sea. In summer again the Jungfrusk~sr water is so much lighter, primarily because of its high temperature, that it can appear in the Bothnian Sea as surface water only, its flow following the line of the Finnish coast (Palmen ^1930).

IV. The water flow in the upper layers of the Bothnian Sea

The deeper trench of the Bothnian Sea, being more than 100 in deep, begins north of the Åland Archipelago and continues as a sickle-shaped depression curving north and west and south. The greatest depth of 254 in is found off Ulvö. In the middle of the Bothnian Sea a dividing ridge extends from the Bight of Gävle northwards as Finngrundet and other banks, across the 62nd latitude at depths of 40 m to 50 m.

In connection with the spring and fall cruises of the R/V Aranda in 1962 and 1963, special attention was paid to the in- fluence of southern waters penetrating through the Åland Sea and Archipelago Sea into the Bothnian Sea. In the following, the spring conditions of 1963 will be elucidated, mainly by reference to the section Ulvö- Storkallegrund.

June 15, 1963, the temperatures of the uppermost layer, above 15 m, of the section ,nere already around 8°C, in some places as high as 10°C (Fig. 15) as a result of branching of surface currents.

Water of 0°C was found close to the Swed- ish coast at depths of 20 to 40 in. This water is obviously what remains of the cold water of the previous winter. Mid-sea., further to the east, at F 23, a temperature of 2.40°C was measured at 40 m, the temperatures above and below being 1.50°C and 1.60°C, respectively. Closer to the Finnish coast, at F 22, another branch of the coldest winter water was found, with the lowest observed temperature -0.3°C.

Some conclusion as to the history of the relatively warm water at 40 m, F 23, may be reached by looking into the amount of dissolved oxygen in it (Fig. 17). The cold

winter water at depths of 15 to 40 in at F 24 B, F 24, F 23 E and F 22 has a rather high oxygen content because of its earlier contact with the atmosphere during the fall turnover. During the winter the oxygen con- tent has decreased slightly; the saturation percentage was found to be about 80 or 90 (Fig.18). On the other hand, the water at 40n depth at F 23 shows an absolute maximum of oxygen concentration connected with the secondary temperature maximum. This can be interpreted only as water of r>young age», originating from the surface somewhere.

(It is interesting to observe that determin- ation of the absolute concentration of dis- solved oxygen seems, for our purpose, to be somewhat more useful than the saturation percentage, as the temperature changes of the water masses have a greater influence on the degree of oxygen saturation than the bio- logical effects in the cases presented here.)

An overall representation of the tempera- ture conditions in the Bothnian Sea shows the effect of the spring warming, especially in the south (Fig. 19). At the Finnish end of the section Söderhamn-Rauma the whole water column is already warmer than 2°C to a depth of almost 60 m. (A similar effect of warming would have been found close to the Finnish coast in the section Brtimö- Kaijakari, too, had the cruise program per- mitted sampling at the easternmost end of the section.) Near Brämö water somewhat, warmer than the surrounding water is found at 40 in at F 26 h, but not at F 26 g or F 26 i.

This may be the same body of water as that found at F 23, here moving southward be- tween the Swedish coast and the mid-sea ridge.

3 6464-64

is

100 120 140

160 180

200 220

20 40 60 80 100 120 140

160 180 200 220 Fig

• 0 1.

g

^{•• u1l11l1 1} _nqy..._=__'ee_•=c5: \ v _•

'i~~ii

fTC

0= >10°C 9- 10' O= 8- 9'

=7- 8°

[llIH= 4- 7

M=1- 4'

® =05- 111111111

1`

=0.25-0.5

= 0 -0.25

® _ <o

Ulvö Storka1le9rund

F248 F24

SBO'

L \ {{{tP

U

6.20

640

1

^.

D}4ll'! U

-

IIIl 111111 III

0=<5.20

=5.20-5.40

IIII

^6.6 0=5.40-5.60

llIjIIjt 0=5.60-5.80

.

II jSV

Qllffl1 = 5.80- 6.00

=6.00-6.20

- ® = 6.20 6.40

111111111 = 6.40- 6.60

= >6.60

s. 15-16. Te» aperature 0)1(1. salinity Oil. the section Utvö Stab(lllegI Ir)zd, o/ the Bothllian Sea., Julle 15, 1963.

19

Ulvö 1963.06.15 Storkatlegrund

20 40 60 80 100 120 140 160 180 200 220

F24B F24 F23 F23 F22 F

20 / -- --110--- / / /

40 //// //, /

60 / // / 9o' /// / / / ///

// //~ / / / / /

/'

100 / i/ /

eo/k/

120 140

02 70o

160 O = >100

, =90-100

180 ~ M =80- 90

®='70-80 200

220 L

Pigs. 17-18. Dissolved oxygen on the section Ulvö - ,Storlrallegrud o/ the Bothnian Sea, June 15, 1963, in mi/l

and as saturation perceratage.

UMEÅ

SEA OF BOTHNIA

1963. VI. 10-14

_ULVÖ

,ae`

^FZ z3

i

^lls

VAASA 'KASKINEN

STORKAI(EQRUND"

BR,4MÖ _KAIJAKARI

MÄNTYLUOTO

SÖDERHAMN

F32 F.

lo 20 30 - 40 50 60 70 80 90 100 110 120

RAUMA

LIII

^{= 12-10°C}

= 9-8°

—:= 8-7°

= 7-4°

m

^{= 4-1°}

® = 1-05,

® = 0,5—Q25°

= Q25-0°

Fig. 19. Temperature distribution in Otte Bothnian Sea, June 10-14, 1963.

20

21

SEA OF BOTHNIA

1963 VI. 10-14

_ULVÖ

BRÄMÖ ^{I 9 b}

UMEÅ

/ c

^VAASA

F24b F27 23E

F22

F21 F21a

KASKINEN

9 / BGO

9 ~ ~ ~ S10RKALLEGRUND

G G ~i~ ~g~' KAIJAKARI

MÄNTYLUOTO

so /

X80.

SÖDERNAMN w

RAUMA

02 m//(

9.50-10.00

=9.00-9.50 8.50 - 9.00

=8.00-8.50 2 = 7.50-8.00 L] = <7.50

Fig. 20. Amounts of dissolved, oxygeia, in nnl/l in the Bothnian Sea, June 10-14,1963.

RA IJAKARI

MANT YL UOTO

= >10'C

"F k P^RAUMA = 10-9"

= 9-g.

ni

^{'iii'= 8 - 7 *}

[~ fN7 =025-0°

SÖDERHAMN

10 20 30 40 50 60 70 80 90 100 110

BRA

F3 F31[

22

A study of the oxygen concentration in the Bothnian Sea (Fig. 20) shows the amounts of dissolved oxygen to be higher near the Finnish coast than near the Swed- ish coast, notwithstanding the lower tem- perature in the latter region. This, too, is consistent with the hypothesis of a body of water coming from the south in spring.

Thus, at a depth of 30 to 40 in a relatively warm current seems to flow in spring, fol- lowing the course of the deeper part of the Bothnian Sea, turning at the latitude of

Storkallegrund and Ulvö around the north- ern end of the mid-sea ridge, and then fi- nally southwards on the Swedish side of the Bothnian Sea.

The summer development then leads to the situation found in early fall, which is indicated by the oceanographic observa- tions of August 28 to 31 (Fig. 21). The sur- face layer, extending to a depth of about

25 m, is as 15° to 16°C. The lower end of the summer thermoclino reaches temperatures as low as 2° to 3°C, except at the northern

HUSUM 2 r i RÖNNSKAR

SEA OF BOTHNIA

1963. VIII. 28-31 ULVÖh":' II

~z,azuil

KASKLNEN

~~ I~1 fi`~~%~ r -~

I

^{~ ~/ S} TORKA LIEGRUND • Illll."~~~~`(~~~~{~I7*?-.. -:.1 -____ Ila

I

120

Pig. 21. Temperature distribution in the Bothhtiaaa Sea, Awgzust 28-31, 1963.

23

end of the Bothnian Sea. The section south in summer turns around and returns Husum-Rönnskär still crosses a body of southward from the Storkallegrrid-U1vö.

rather cold water. This is another indication Thus the general circulation pattern is the that the warmer water coming from the same in early fall as in spring.

V. The flow over the Quark to the Bothnian Bay The Bothnian Bay consists of a single

basin, reaching depths of 100 to 120 in northeast of Bjuröklubb. The Bothnian Bay is separated from the Bothnian Sea by the Quark, a set of openings in an elevation, a large part of which rises above the sea sur- face. The main opening, in the middle of the Quark but often called the East Quark, has a sill depth of only 10 to 15 in at Nordvalen.

The Western Quark, between the island of Holmö and the Swedish mainland, has a deep channel with a depth of about 30 ra, but measurements have so far not disclosed any appreciable exchange of water through this sound.

A still narrower channel passes through the Vaasa sherries to Ritgrund. Strong cur- rents are frequently observed at its nar- rowest points; however, its narrowness and small depth of about 10 in do not allow the passage of large water masses. The main exchange of water between the Bothnian Sea and the Bothnian Bay takes place through the Quark proper, — through the two or three deeper channels and through the many wide openings with a mean depth of only a few meters.

To start with, the exchange of water through the Quark was studied by com- paring the density of water at the light.-

2.00 Qt

3.00

4.00

.Pig. 22. The changes ina the tleasitg of surface water at »Snaipaia> in the Quaric awl of deep water at »Helsinagkallann> in the south of the

Bothnian Bay.

25

ships Snipan and Helsingkallan (Fig. 22).

The comparison was made for the year 1925, as the lightship Helsingkallan was with- drawn shortly afterwards. To eliminate freak phenomena at the surface, the tem- perature and salinity values observed at the depth of 5 m by Snipan were chosen to represent the surface layer and the density was computed from these. At Helsingkallan the depth of 30 m close to the bottom was chosen.

The result shows the surface water at Snipan to have a greater density than the bottom water at Helsingkallan during the winter until May 1. On the next observation day, May 11, the density difference is al-

ready reversed and the surface water of Snipan cannot any longer sink to the hot- tom after passing the Quark. This situation holds until the observations on September 21. Thus the deeper part of the Bothnian Bay is deprived of the sinking warmer water from the south at a much earlier date than the Bothnian Sea, in which the water from the Archipelago area sinks until the end of May.

An extensive investigation of the early spring conditions in the Bothnian Bay was planned for 1962, when the cruise with the R/V Aranda was scheduled to cover the Bothnian Bay during the first days of May shortly after the break-up of the ice.

BAY OF BOTHNIA 1962 VI. 18-20

BJURÖKLUBB

_ >100C

Elli =

Uj = 9-8°

'iii' = 8 - 7°

®= 7 -4°

® = 4 - 2°

®= 2 - 1°

® = I -05

® = 05-025

=025-0°

®= 50°

L'ig. 23. Temiaperatnie di$tiibutioIi in the Bodanian Bay, Jwne 18-20, 1902.

SIKEÅ

10 20 30 40 50 60 70 80 90

4 ^6464-64

1962 09.12

S4 S3 S2 Si

MÄSSKÄR

50

100

S ⁰

o0

0

SIKEA 55 S4 _MÄSSKÄR

I I \ I I I~~ I

-

50

26

0

SIKEA S5

foc

Fit's.

24-25. Temperature and salinity on the section Sikeå — Dlässlcär o/ the Bocknian.

Bay, September 9, 1962.

02~o0

O SIKENS S4 53 52

Si MÅSSKÅR

1962 09. 12

S4 S3 S2 S1 MÄSSKÄR

0 SIKENS 02

50

02 mi/l

]= <7.50 in =7.50-8.00

= 8.00-8.50 [[] = 8.50 —9.00

=9.00-9.50

100

02 %o

>105

= 100 —105 0= 95-100

® 90 — 95

100

27

Figs. 26-27, Aviounts o/ dissolved oxygen, itii m111 and as saturation percentage, in the section Sikeå — Dlässkär o/ the Bothnian Bay, September 9, 1962.

28

Unfortunately, the cruise was delayed and readily identified in the section Sikeå- the waters had warmed up appreciably by Mässkär (Figs. 24-25). At station S 3 the the time Aranda reached the area in June temperature at 50 in was 9. 1°C and at 40 in (Fig. 23). The temperature of the surface 3.0°C. The cold water at intermediate water was close to 10°C and the cold winter depths may have come through the Quark water in the depths had apparently been in spring. The warm water close to the bot- replaced by warmer, saline water from the tom cannot have been many days old, to

south. judge from the well-defined interface be-

The Aranda cruise of fall 1962 was accom- tween the water masses. Obviously, the rel- panied by persistent strong southwesterly atively deep water, with high temperatures, winds which drove warm, rather saline wa- contains a smaller amount of dissolved oxy- ter over the sill of the Quark. This water was gen than the cold spring water (Figs. 26-27).

BAY OF BOTHNIA

1962. IX. 13-15

lom-

FINNKLIPPA å

C-.'- ; I , KEMI

2 zl

RQNNSKAR

• KC-LM!

ç1J

-TT

R3 ~R4 RS

RR

RAAHE

BJURÖKL UBB

b/ 7 BB OHTAKARI

SIKEA

sa s ç 1 1 ' = iz-10°C

UMEÅ _

' PIETARSAARI

M=

^{= 9 - s e - 7°}

:•F17 :• F16 F15 ~ ® = 4 - 1'

lo— RITGRUND

zo _

30

40 ---- —• = 0,25- 0°

Pig. 28. Temperature distribution in the Bothnian Bay, September 13-15, 1962.

O = 0 9'

= 9-B°

= 8-7'

l

fill = 2- 1'

® = i -0w- ii =oso-O2s 025-ö

29

Farther northward in the Bothnian Bay (Fig. 28) the fresh intrusion is found at a depth of 40-60 m at station B 5 of the ° section Bjuröklubb-Ohtakari, where it is closer to the Finnish coast. The same water is not found at any station of the section Röm7skär-Raahe. Either the Varm water had not had enough time to penetrate so ₅₀ far northward or it had floraed westward and then southward along the Swedish coast in a kind of cyclonic eddy. The latter as- sumption is supported by other observa- tions.

to

1963.09.01-02 55 54 53 52

The fall cruise of 1963 was made later than usual, in September. This time, too, a warm water mass had been driven over the sill. It was readily identified on the section Sikeå-Mässkär, but was not found farther northward, at least not on the section Rönn- skär-Raahe or Finnklippan-Kelmi. Because a small storm arose during the cruise, it was decided to rerun the section Sikeå-Mässkär (Fig. 29). Now the water had already be- come warmer on the Finnish side, which can only have been due to an intrusion of warm- er water from the south. At the same time colder water had arrived at the Swedish end of the section. This could only have come from the north, from the region of Bjurö- klubb. The waters of the Bothnian Bay thus seem to circulate counterclockwise, at least in the fall, in a wind-driven gyre

(Palmen

1930).

The annual march of the oceanographic conditions of the Bothnian Bay may be further studied by a scrutiny of the meas- urements at a single station. Thus at station B 5 of the section Bjuröklubb-Ohtakari the whole water column cools off in winter (Figs. 30-31). However, the spring obser- vations made on board the R/V Aranda point to a body of deep water below 100 m in the middle of the Bothnian Bay which does not cool below 1° or 2°C.

At the station R 6 of the section Rönn- skär-Raahe (Figs. 32-33) the water near the bottom shows only a very minor annual range of temperature.

10o -

t0

1963.09.05 Sikeå 55 54 53 5

100

4 to

50

100'—

Fig. 29. Changes

oj

temperature on the section Si/ceå

—

.Mässkär

of the Bothnian Bay, between September 2 and September 5, 1963.

30

50

1961 1962 1963

0 viu ix x x1 x11 I II III W V VI v11 v 11 Ix x XI XII I. i u i iii I i„ 1 liibl / ₁\\\ ^>f10 I llay l 1 //

11 1

111 ~, I,(

III I,Y II Al

\\\

1 \'\\S \ 11111 I V 1 \ 1 11111 1 1 / /

ii ', '? B AY OF BOTHNIA

1'i f ' \ 11 n i r r ₁₁₁ I if o

[1/

~` 'I ~ ~, 85

;1

~1 ::f>7

t_°C

ill I . li `'1 `' 1 11, Iii

V'////

. 111 ^{1 11} rll,11 ^{I 11} 11 1 v/

50

1961 1962 1963

0 vm ^{Ix x x1 xl1}

I

^{I it m 1v v} VI v11 v 11 1x x XI XII II

3.75 ^{I 3.75}

1 <350 `

-' 1 1 ~\

\ 1

`.\ ' \ 3.75

BAY OF BOTHNIA

`\ ' —' -

`I - BS

S%o

ue\4.Q0

>4 5\4.00 4f70 >4.25

50

1961 1962 1963

0 VIII ix x X1 XIS

I

^{i 11 111 Iv V VI VII VIII} _<3. ^{ix x XI XII}

I

^{i 11}

400 BAY OF BQTHA'/A

' R6

i

4.00

50

1961 1962 1963

0 vill Ix x x1 x11 l 11 Ili Iv v vi VII vill ^{Ix x x} XII ^{1 11}

2 o /i+o'i' ,'~ BAY OF 80THNIA

Fig. 30-31. Temperatine and salinity fluctuations at Station B 5, in the Bothniay. Bay, 1961 to 1963.

Fig. 32-33. Temperature and salinity /lvctuations at Station R 6, in the northern Bothnian Bci.y, 1961 to 1963.

31

Fig. 34-37. T7o:titaI distribution oj te)nperatvre, salinity and the dissolved oxygen, at va>iovs times, at Stations F 69 in the Lägskär Deep; at PÅ 4 in the Åland Sea; at F 30 in the Bot/inian Sea and at

F 18 in the Quark.

References

GRANQVIST, GUNNAR,

1936: Zur Kenntnis der Temperatur und des Salzegehaltes des Baltischen Meeres and den Klisten Finnlands. Fennia 65. Helsinki.

HELA, IL➢IO,

1958: A hydrological survey of the waters in the Åland Sea. Geophysica 6: 3-4. Helsinki.

LISITZIN, EUGENIE,

1946: The relations between wind, current and water level in the Gulf of Bothnia. Soc. Sc. Fennica, XIII. 6. Helsinki.

—»— 1951: A brief report on the scientific results of the hydrological expedition to the Archipelago and Åland Sea in the year 1922, Fennia 73, No. 4. Helsinki.

MERENTUTIK. JULIO.

No. 200: Icewinter 1960/61 along the Finnish coast. Helsinki 1961.

—»— No. 201: Temperature and salinity at the fixed Finnish stations 1957-1959. Helsinki 1962.

—»— No. 206: Ice winter 1961/62 along the Finnish coast. Helsinki 1962.

—»— No. 208: The Gulf of Bothnia in winter time. I. Data from winter cruises 1959-63.

Helsinki 1964.

—»— No. 213: Ice winter 1962/63 along the Finnish coast. Helsinki 1964.

PALMMIN, ERIK,

1930: Untersuelmngen fiber Ilie Strömungen in elen Finnland umgebenden Meeren. Soc. Sc. Fennica V. 12. Helsinki.

SlnuOaoiu,

HEIKKI,

1946: On the temperature and salinity of the sea in the vicinity of the Bogskär lighthouse in the Northern Baltic. Sec. Sc. Fennica, XIII. 7. Helsinki.

SJönLOis, VEIKKO,

1961: Wanclerungen des Strömlings (Clupea Harengus L.) in einigen Schären- und Hochseegebieten der Nördlichen Ostsee. Vanamo, 23, No. 1. Helsinki.

WITTING, ROLF

und

PETTERSSON, HANS,

1925: Thalassologische Beobachtungen im Alanclsmeer und Schdrenmeer in Juli 1922 und Juli 1923. Merentutk. Julk. N:o 30.

Helsinki.

WITTING, ROLF,

1908: Untersuehungen zur Kenntnis der Wasserbewegungen und der

Wasserumsetzung in den Finnland umbenden Meeren, I. Finnliinclische Hyclro-

graphische Untersuchungen. No. 2. Helsinki.